Add command to link a different manifest

[visigoth]

Adds pore manifest link <name> to link a named and existing manifest file as the current manifest. One can then pore sync -d to sync to that manifest's revisions. Just a bit nicer than having to manage the symlinks yourself.

[jmgao]

@yoelglus moved us to the latest version of clap using the derive macro interface, and it's not immediately obvious to me how to do this with that interface.

That being said, I think this should probably be in pore init instead, for symmetry with rerunning repo init inside an already existing tree?

[visigoth]

personally i always hated that repo shoved this into the init command, since it doesn't feel like an initialization procedure. doing this via repo init always made me wary that i am somehow re-initializing the repository and would need to use all the options i used the first time to set it up in order to have the appropriate behavior.

[yoelglus]

@visigoth - I agree repo init is far from intuitive, but the question is what we are trying to achieve with pore? If I understand correctly, this is mostly about speed. Changing the API will raise the bar to migrate from repo to pore.

[visigoth]

whynotboth?

[yoelglus]

@visigoth - my personal opinion here is that having more than one way to do this can lead to some unintended consequences:

1. Confusion as to what is "the right way" to do it.
2. We will have to maintain both moving forward.
3. Diverting from repo's API will introduce a barrier for people to migrate. If repo becomes faster in the future, I don't see a reason not to deprecate pore. That will be harder for people that got used to these subtle differences.

[yoelglus]

But in general - @jmgao being the maintainer - I wander what you think here 😄 .